1. Field of the Invention
This invention relates to a lens position adjusting method suited for flange back adjustment of an inner focus lens assembly and a video camera using such a lens position adjusting method.
2. Description of the Prior Art
In an inner focus lens assembly whose front lens is fixed, when zooming is changed by moving the variator, the focal position for a focus lens varies with movement of a zoom lens. To cope with the variation, in existing video cameras using such inner focus lens assembly, information on changes in focal position for the focus lens with movement of the variator is stored in advance in a memory as mapping data. A curve indicating changes in focal position of the focus lens with movement of the variator is called a tracking curve.
FIG. 1 is an example of a zoom tracking curve. In FIG. 1, the abscissa indicates the position of the variator, and the ordinate indicates the position of the focus lens. When the variator is moved, focalization can be maintained if the focus lens is moved in accordance with the tracking curve.
In the existing video cameras equipped with such an inner focus lens assembly, a table is prepared in accordance with the tracking curve. When the variator is moved, the position of the focus lens is controlled in accordance with the data in the form of the tracking curve stored in the memory. Thereby, even when the variator is moved, focalization is maintained.
Because of a lens mounting error or the like, there arises a difference between an actual tracking curve and the designed tracking curve stored in advance as mapping data. For correcting the difference, flange back adjustment is executed in the existing video cameras having an inner focus lens assembly.
The existing flange back adjustment is performed as follows. When the variator is first set to a position corresponding to a point of inflexion as designed, the position of the focus lens is controlled so that the focus lens moves to the focal position to detect the point of inflexion. When the focus lens reaches the focal position, the focus lens is moved by a designed moving distance of the focus lens from the point of inflexion to the telephoto end. When the focus lens reaches the telephoto end, the variator is moved to the focal position, and the position is provisionally stored as a telephoto end.
The variator is moved to the point of inflexion, and the focus lens is controlled to a focal position. The present focal position of the focus lens is compared with the previous focal position to determine whether a difference between the previous and present focal positions for the focus lens comes within the predetermined value. The process is repeated, until the difference between the previous and present focal positions for the focus lens comes within the predetermined value. When the difference between the previous and present focal positions for the focus lens comes within the predetermined value, the position is determined as the telephoto end.
Then, the focus lens is moved from the wide end to the telephoto end by a designed moving distance for the focus lens. The variator is moved to a focal position, and the focal position is determined as a wide end.
As mentioned above, in the existing flange back adjustment, the variator is moved to a focal position, and the focal position is determined as a telephoto end. To detect a position for the telephoto end by moving the variator to a focal position, it is necessary to sample data in such a manner as to maintain the accuracy completely within the depth of focus. As a result, it becomes impossible to make flange back adjustment by moving the variator at a high speed.
FIG. 2 is a tracking curve near the telephoto end. To detect focalization, it is necessary to move a lens within the depth of focus. Therefore, in FIG. 2, to obtain accuracy within fd, where fd is the depth of focus, it is necessary to obtain sampled values within accuracy of distance t.sub.d, when the variator is moved. FIG. 2 shows a case where the variator is moved at a high speed. When sampling timing is constant in this case, distance between samplings t becomes longer as the variator is moved faster, which makes it impossible to keep the distance between samplings t within the distance t.sub.d. As a result, in the existing method, some limitations reside in the moving speed of the variator in the case of the flange back adjustment.